Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2013/iii/paper-22/3/a/solution

An elliptic curve has good reduction of an elliptic curve at if it admits a Weierstrass equation of an elliptic curve over whose reduced projective cubic is nonsingular. Equivalently its minimal discriminant is a -adic unit.
For the given integral short equation,
It therefore directly supplies good reduction outside . These three primes cannot be rescued by changing the model. An admissible change of Weierstrass model changes the discriminant by a twelfth power, so its valuation changes by a multiple of twelve. The displayed valuations are not congruent to zero modulo twelve. No integral model can have unit discriminant at any of those primes. The good primes are exactly .

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